Joining different UVmaps from different objects
You can't combine the UV maps of multipe objects in Blender. Officialy the the TextureAtlas plugin only supports lightmaps (the link could be outdated).

But wait Gleb Alexandrov made a promising tutorial:


  John Carmack from id Software used megatexture. You use megatexture.


game-level-texturing-texture-atlas

Related Unity Links:


Draw Call Minimizer
whats-the-best-way-to-reduce-draw-calls